Radiology Lecture 11.docx

Lecture 11 Chapter 3 – Radiation Biology Some x-rays do not reach the x-ray film; they are absorbed by the patient’s tissue. All ionizing radiations are harmful and produce biologic changes in living tissues. Ionization occurs when x-rays strike the patient’s tissue. The damaging biologic effec...

Lecture 11 Chapter 3 – Radiation Biology Some x-rays do not reach the x-ray film; they are absorbed by the patient’s tissue. All ionizing radiations are harmful and produce biologic changes in living tissues. Ionization occurs when x-rays strike the patient’s tissue. The damaging biologic effects of x-radiation were first documented shortly after the discovery of x-rays. Since that time, information about the harmful effects of high-level exposure to x- radiation has increased based on studies of atomic bomb survivors, workers exposed to radioactive materials, and patients undergoing radiation therapy. Although the amount of x-radiation used in dental imaging is small, biologic damage does occur. Free radical formation occurs when an x-ray photon ionizes water, the primary component of living cells. Ionization of water results in the production of hydrogen and hydroxyl free radicals. A free radical is an uncharged (neutral) atom or molecule that exists with a single, unpaired electron in its outermost shell. A free radical is a neutral atom or molecule that exists with a single, unpaired electron in its outermost shell. Formed when an x-ray photon ionizes water. Cell damage occurs primarily through the formation of free radicals. Ionization of water is the MOST COMMON mechanism of damage in humans. It is highly reactive and unstable; the lifetime of a free radical is approximately 10 seconds. To achieve stability, free radicals may: recombine without causing changes in the molecule combine with other free radicals and cause changes, or combine with ordinary molecules to form a toxin (e.g., hydrogen peroxide [H2O2]) capable of producing widespread cellular changes Damage to living tissues caused by exposure to ionizing radiation may result from a direct hit and absorption of an x-ray photon within a cell or from the absorption of an x-ray photon by the water within a cell accompanied by free radical formation. Free radical formation created by the ionization of water may occur. Steps of hydrogen peroxide formation: X-ray photons are absorbed by water Free radical formation Free radicals combine Cellular dysfunction and biologic damage The direct theory of radiation injury suggests that cell damage results when ionizing radiation directly hits critical areas (thru direct contact) within the cell. Direct injuries from exposure to ionizing radiation occur infrequently; most x-ray photons pass through the cell and cause little or no damage. The indirect theory of radiation injury suggests that x-ray photons are absorbed within the cell and cause the formation of toxins, which in turn damage the cell. When x-ray photons are absorbed by the water within a cell, free radicals are formed. The free radicals combine to form toxins (e.g., H2O2), which cause cellular dysfunction and biologic damage. An indirect injury results because the free radicals combine and form toxins, not because of a direct hit by x-ray photons. Simply put, toxins are created from the formation of free radicals. The dose-response curve is used to correlate the damage to tissue with the dose of radiation received. When dose and damage are plotted on a graph, a linear, non-threshold relationship is seen. A linear relationship indicates that the response of the tissues is directly proportional to the dose. Linear relationship indicates that the response of the tissues is directly proportional to the dose. A non-threshold dose-response curve suggests that no matter how small the amount of radiation received, some biologic damage does occur Consequently, there is no safe amount of radiation exposure. In dental imaging, as mentioned earlier, although the doses received by patients are low, damage does occur. Stochastic effects occur as a direct function of dose. The probability of occurrence increases with increasing absorbed dose; however, the severity of effects does not depend on the magnitude of the absorbed dose. Examples of stochastic effects include induction of leukemia and other cancers (i.e., tumors). Simply put, stochastic biologic effects from radiation occur as a direct result of exposure, but severity is not determined by dose. Sequence of Radiation Injury A latent period can be defined as the time that elapses between exposure to ionizing radiation and the appearance of observable clinical signs. The latent period may be short or long, depending on the total dose of radiation received and the amount of time, or rate, it took to receive the dose. The more radiation received and the faster the dose rate, the shorter the latent period. The latent period is the time that elapses between exposure to ionizing radiation and the appearance of observable clinical signs. After the latent period, a period of injury occurs. A variety of cellular injuries may result, including cell death, changes in cell function, breaking or clumping of chromosomes, formation of giant cells, cessation of mitotic activity, and abnormal mitotic activity. Examples include: burning sore throat, difficulty swallowing, red skin The period of injury is a variety of cellular injuries may result. The last event in the sequence of radiation injury is the recovery period. Not all cellular radiation injuries are permanent. With each radiation exposure, cellular damage is followed by repair. The recovery period is the last event in the sequence of radiation injury; depending on a number of factors, cells can repair the damage caused by radiation. The cumulative effects of repeated radiation exposure can lead to health problems (e.g., cancer, cataract formation, or birth defects). Radiation effects are additive. Determining Factors of Radiation Injury Total dose: the total amount of radiation energy received or absorbed. More damage occurs when tissues absorb large quantities of radiation. Quantity of radiation received. Dose rate: Rate at which exposure to radiation occurs and absorption takes place (dose rate = dose/time). More radiation damage takes place with high dose rates because a rapid delivery of radiation does not allow time for the cellular damage to be repaired. A PERSON THAT EXPERIENCES ONSET OF SYMPTOMS QUICKLY HAS AN INCREASED DOSE AND DOSE RATE; THESE DECREASE THE LATENT PERIOD. Cell sensitivity: More damage occurs in cells that are most sensitive to radiation, such as rapidly dividing cells and young cells. Age: children are more susceptible to radiation damage Short-term effects are associated with large amounts of radiation absorbed in a short time (e.g., exposure to a nuclear accident or the atomic bomb). Acute radiation syndrome (ARS) is a short-term effect and includes nausea, vomiting, diarrhea, hair loss, and hemorrhage. Short-term effects are not applicable to dentistry. Effects that appear after years, decades, or generations are termed long-term effects. Long-term effects are associated with small amounts of radiation absorbed repeatedly over a long period. Repeated low levels of radiation exposure are linked to the induction of cancer, birth abnormalities, and genetic defects. Seen after years, decades, or generations. Somatic cells are all the cells in the body except the reproductive cells. The reproductive cells (e.g., ova, sperm) are termed genetic cells. Somatic effects are seen in a person who has been irradiated. Radiation injuries that produce changes in somatic cells produce poor health in the irradiated individual. Major somatic effects of radiation exposure include the induction of cataracts and cancer, including leukemia. These changes, however, are not transmitted to future generations Genetic effects are not seen in the irradiated person. Passed on to future generations. Genetic damage cannot be repaired. Reproductive cells (e.g., ova, sperm) Not all cells respond to radiation in the same manner. A cell that is sensitive to radiation is termed radiosensitive; one that is resistant is termed radioresistant. The response of a cell to radiation exposure is determined by the following: Mitotic activity: Cells that divide frequently or undergo many divisions over time are more sensitive to radiation. Cell differentiation: Cells that are immature or are not highly specialized are more sensitive to radiation. Cell metabolism: Cells that have a higher metabolism are more sensitive to radiation. Cells that are radiosensitive include blood cells, intestinal cells, immature reproductive cells, and young bone cells. The cell that is most sensitive to radiation is the small lymphocyte. Radioresistant cells include cells of bone, muscle, and nerve A critical organ is an organ that, if damaged, diminishes the quality of a person's life. Critical organs exposed during dental imaging procedures in the head and neck region include the following: Thyroid gland Bone marrow Skin Lens of the eye Traditional System Roentgen (R) Radiation absorbed dose (rad) Roentgen equivalent (in) man (rem) SI System Coulombs/kilogram (C/kg) Gray (Gy) Sievert (Sv) The radiation absorbed dose, or rad, is the traditional unit of dose. 1 Gray = 100 rad The SI unit equivalent of the rem is the sievert (Sv). Units of measurement Exposure Coulombs/kg – amount of radiation in the air Dose Gray (Gy) – amount of radiation absorbed by the tissue Dose equivalent Sievert (Sv) – measurement of the effect on the tissue The cosmic exposure depends on the elevation above sea level; the higher the altitude, the more exposure to cosmic rays. Terrestrial exposure comes from the ground; an example includes naturally occurring uranium-enriched soil. In the United States, the average person is exposed to approximately 3.1 mSv of background radiation per year. About 50% of the radiation received in a lifetime is from background radiation. Consumer products (e.g., luminous wristwatches, televisions, computer screens), fallout from atomic weapons, weapons production, and the nuclear fuel cycle are all sources of human- made radiation exposure. Medical radiation is the greatest contributor to human-made radiation exposure (0.00053/Sv/yr). The potential risk of dental imaging inducing a fatal cancer in an individual has been estimated to be approximately 3 in 1 million. The risk of a person developing cancer spontaneously is much higher, or 3300 in 1 million. A 1-in-1-million risk of a fatal outcome is associated with each of the following activities: riding 10 miles on a bike, 300 miles in a car, or 1000 miles in an airplane. These risk estimates suggest that death is more likely to occur from common activities than from dental imaging procedures and that cancer is much more likely to be unrelated to (dental) radiation exposure. In other words, the risks from dental imaging are not significantly greater than the risks of other everyday activities in modern life. THE RISK OF DENTAL RADIOGRAPHY CAUSING FATAL CANCER IS ABOUT 1/1,000 THE RISK OF SOMEONE SPONTANEOUSLY DEVELOPING CANCER. With dental imaging procedures, the critical organs at risk include the thyroid gland and active bone marrow. The skin and eyes may also be considered critical organs. A total of 250 rad (2.5 Gy) in a 14-day period causes erythema, or reddening, of the skin. To produce such changes, more than 500 dental films (F-speed film, exposure rate 0.7 R/second) in a 14-day period would have to be exposed. This scenario does not occur in dental imaging. Receptor choice: Radiation exposure can be reduced by using digital sensors. The use of sensors can reduce exposure time by 50% to 90% when compared to conventional radiography. Radiation exposure can be limited by using the fastest film available. The use of F-speed film instead of D-speed reduces the absorbed dose by 60%. Collimation: Radiation exposure can be limited by using rectangular collimation. The use of rectangular collimation instead of round collimation reduces the absorbed dose by 60% to 70%. Technique: Radiation exposure can be limited by increasing the target-receptor distance. The use of the paralleling technique and increased target-receptor distance reduces the skin dose. WHAT COLLIMATION AND RECEPTOR COMBINATION WOULD RESULT IN THE LOWEST RADIATION DOSE FOR A PATIENT? RECTANGULAR COLLIMATION WITH A DIGITAL SENSOR Chapter 11 – Film Mounting Film mounting is an essential step in the interpretation of dental radiographs. The dental radiographer must be able to mount dental radiographs in correct anatomic order. Mounted radiographs, or radiographs placed in a film holder in anatomic order, are essential to the dental professional. Anatomic order refers to how teeth are arranged within the dental arches. In dental radiography, film mounting is the placement of radiographs in a supporting structure or holder. A film mount is a cardboard, plastic, or vinyl holder that is used to support and arrange dental radiographs in anatomic order A film mount may be opaque or clear. An opaque film mount is preferred because it masks the light around each radiograph. Subtle changes in density and contrast are easier to detect when extraneous light is eliminated. To minimize extraneous viewbox light, each window of a film mount should contain a radiograph. When all the windows are not filled with radiographs, black opaque paper can be placed in the unused frames. The overall size and shape of the film mounts are designed to fit a variety of viewboxes found in the dental office; the size of the film mount should correspond to the size of the viewbox. Any trained dental professional (dentist, dental hygienist, dental assistant) with knowledge of the normal anatomic landmarks of the maxilla, the mandible, and related structures is qualified to mount dental radiographs. The dental radiographer should always mount films immediately after processing. Mounted radiographs are easily stored in the patient record and are readily accessible for interpretation. The dental radiographer should label the film mount before the films are mounted. A special marking pencil designed to write on paper, plastic, or vinyl can be used to label film mounts. Radiographs are easily identified when the film mount has been clearly and legibly labeled with the following information: Patient's full name Date of exposure Dentist's name Radiographer's name The patient's name and date of exposure are essential It is important to note the curve of Spee when mounting bite-wing radiographs. The anterior-posterior anatomic curvature of the occlusal surfaces of the teeth, or curve of Spee, begins at the tip of the lower canine and follows the buccal cusps of the posterior teeth to the anterior border of the ramus. The curve of the maxillary arch is rounded or convex, while the curve of the mandibular arch is caved in or concave. The film is positioned in the packet such that the raised side of the dot faces the x-ray beam during exposure. The identification dot is used to determine film orientation (i.e., determining the patient's right and left sides). After processing, the films should be placed in the film mount so that all the embossed dots are either raised (labial mounting) or depressed (lingual mounting); all the embossed dots must face in the same direction. The dental radiographer can then distinguish between the right and left sides of the patient. Either labial or lingual mounting can be used. Labial mounting is the preferred method of mounting dental radiographs and is recommended by the American Dental Association. In the labial mounting method, radiographs are placed in the film mount with the raised (convex) side of the identification dot facing the viewer (dental radiographer). The radiographs are then viewed from the labial aspect (thus the term labial mounting). With this method, the radiographs are viewed as if the viewer is looking directly at the patient; the patient's left side is on the viewer's right, and the patient's right side is on the viewer's left Lingual mounting can be used as an alternative method. Although some practitioners still use lingual mounting, this system of film mounting is not recommended. In the lingual mounting method, radiographs are placed in the film mount with the depressed (concave) side of the identification dot facing the viewer. The dental radiographer then views the radiographs from the lingual aspect (thus the term lingual mounting). With this method, the radiographs are viewed as if the dental radiographer is inside the patient's mouth and looking out; the patient's left side is on the viewer's left, and the patient's right side is on the viewer's right Helpful Hints for mounting radiographs: DO master the normal anatomy of the maxilla, the mandible, and adjacent structures. A working knowledge of normal anatomy is necessary to correctly mount films. DO label and date the film mount before mounting the films; always include the patient's full name, the date of exposure, the dentist's name, and the radiographer's name. DO mount films immediately after processing. DO mount radiographs in a designated area; use a light-colored working surface in front of a viewbox. DO use an opaque film mount to block out extraneous light around each film. DO use clean dry hands to mount radiographs, and hold each radiograph by the edges only. DO identify the embossed dot on each film; always mount radiographs with the raised side of the dot facing the same direction. For labial mounting, all the raised dots must face the viewer. DO sort the radiographs before mounting them. DO use normal anatomic landmarks to distinguish maxillary images from mandibular images. DO use the order of teeth to distinguish the right side from the left side. DO use a definite order for mounting films. For example, begin with bite-wing images, then mount maxillary anterior periapical films, proceed to mandibular anterior periapical films, and then finish with maxillary posterior periapical films and mandibular posterior periapical films. DO mount bite-wing radiographs with the curve of Spee directed upward toward the distal. DO recognize the differences between maxillary and mandibular teeth (e.g., maxillary anterior teeth have larger crowns and longer roots than do mandibular anterior teeth). DO remember that most mandibular molars have two roots, whereas most maxillary molars have three. DO recognize that most roots curve toward the distal. DO verify the following points after mounting the radiographs: all the embossed dots are oriented correctly, the radiographs are arranged in anatomic order, the radiographs are mounted securely, and the film mount is labeled and dated. DO place the mounted radiographs in the patient's file as soon as possible to eliminate the possibility of loss or mix-up. Any trained dental professional (dentist, dental hygienist, dental assistant) with knowledge of the normal anatomic landmarks of the maxilla, the mandible, and related structures is qualified to view dental radiographs. It is the responsibility of the dentist to establish a final or definitive interpretation and diagnosis. PROCEDURE 28-2 Helpful Hints for viewing radiographs: DO use a viewbox to examine radiographs; avoid holding mounted films “up to the room light” to view. DO block out the harsh light on a viewbox that occurs around the edges of the film mount; harsh light must be masked to reduce glare. DO use a magnifying glass to evaluate slight changes in density and contrast in radiographic images. DO view films immediately after mounting. DO view films under optimal viewing conditions whenever possible; use an area free of distractions with dimmed lighting. DO use a definite order for film viewing. To view a CMS, start with the films on the upper left side of the mount, move horizontally to the upper right, down to the lower right, across the lower left, up to the bite-wings, and then view the bite-wings from left to right. DO examine radiographs using the recommended viewing sequence as many times as necessary to evaluate them for (1) unerupted, impacted, and missing teeth; (2) dental caries, pulp size, and pulp shape; (3) bony changes, level of alveolar bone, and calculus; (4) roots and periapical areas; and (5) all areas not previously examined. DO record all radiographic findings in the patient record.